2Q gate fidelity of 99.99-! New architecture brings changes to superconducting quantum computers

Toshiba researchers have made a breakthrough in quantum computer architecture: the basic design of dual transmon couplers will improve the speed and accuracy of quantum computation in tunable couplers. on September 15, the related research results were published as "Dual transmon couplers: fast dual quantum bit gates with residual-free coupling of highly detuned superconducting quantum bits" [1] in the Applied Physics Reviews journal. The simulations show that with this new technique, the double quantum bit gate operation has an accuracy of 99.99% and a processing time of only 24 ns.

01Reducing error rates with couplers

The coupler is the key device that determines the performance of a superconducting quantum computer.

The coupling of quantum bits is done through couplers. Until recently, the main device has been a fixed coupler with constant coupling strength, but now, research attention is turning to the tunable coupler: it is made possible to provide the tunable coupling strength needed to improve performance. Tunable couplers achieve almost contradictory requirements: fast two-qubit quantum gates with strong coupling, and reduction of residual coupling errors by turning off the coupling. Furthermore, the quantum bits used in the calculation are preferably fixed frequency transmon quantum bits: they are highly stable, simple in structure and easy to fabricate.

In addition, the frequencies of the two quantum bits coupled should be significantly different, as this reduces crosstalk errors and is robust to design values that deviate from the quantum bit frequency, thus improving the qualification rate of the device fabrication.

Current techniques can turn off the coupling of transmon quantum bits with similar frequencies, however, when one quantum bit is irradiated by an electromagnetic wave for control, it becomes susceptible to crosstalk errors on the other quantum bit; moreover, current techniques cannot completely turn off the coupling of quantum bits with significantly different frequencies, leading to errors in residual coupling [2].

Concept diagram of superconducting computer

02World's first: dual transmon coupler

Toshiba has recently designed a new dual transmon coupler, the world's first tunable coupler that can fully turn off coupling and operate two quantum bit gates at high speed for two fixed frequency transmon quantum bits with significantly different frequencies.

The new coupler can fully turn on/off the coupling between quantum bits with significantly different frequencies: fully on allows high-speed quantum computation with strong coupling, and then fully off eliminates residual coupling, improving the speed and accuracy of quantum computation. Simulations of the new technique show that it achieves a double quantum bit gate - a fundamental operation in quantum computing that operates with 99.99% accuracy and a processing time of only 24ns.

Circuit diagram of the dual transmon coupler

The dual transmon coupler consists of two fixed frequency transmon quantum bits, and two additional fixed frequency transmon quantum bits for computation. The dual transmon coupler has a ring with three x's representing two transmon Josephson junctions and an additional Josephson junction. The magnetic flux Φex in the loop can be adjusted by an external magnetic field to make the coupling strength between the quantum bits on both sides exactly zero, thus closing the coupling completely. The coupling strength can also be increased to tens of MHz by increasing the magnetic flux to enable fast two-qubit quantum gate operation.

Flux dependence of coupling strength in a dual transmon coupler

03Future: Improved fidelity for high-performance quantum computing

Toshiba's dual transmon coupler can be applied to fixed-frequency transmon quantum bits, achieving high stability and ease of design. It is the first to realize coupling between fixed-frequency transmon quantum bits with significantly different frequencies that can be fully turned on and off, and to provide high-speed, accurate double-transmon gates.

In the future, the technology is expected to drive the implementation of higher performance quantum computers, which will help to achieve areas such as carbon neutrality and new drug development.

The next experimental improvements will focus on two areas: where the relaxation and decoherence of transmon quantum bits will reduce performance. However, judging from its short gate operation time (24 ns) and the recently reported long coherence time of transmon (over 300 μs), the new coupler is expected to achieve high two-qubit gate fidelity. Another important issue is the deviation of the critical current of the Josephson junction from the design value: the impact of the deviation of the critical current on the coupler performance is an important issue for future work [3].

Reference links：

[1]https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.18.034038

[2]https://phys.org/news/2022-09-double-transmon-coupler-faster-accurate-superconducting.html

[3]https://www.businesswire.com/news/home/20220913006341/en/